Exponential decline or increases in biologically active
molecules due to food web interactions

It is well known that organisms are not 100% efficient at processing the food that is captured during feeding. In general there is about a 90% loss of energy in the process of capture, assimilation, and energy allocation towards organismal growth, which means that by default there is only a 10% efficiency of energy transfer between trophic levels. The "transfer efficiency" therefore is simply the ratio of the energy added to the upper trophic level, measured in terms of grams of organic carbon, per unit volume (or area), per unit time, over the energy provided by the lower trophic level. Given this inefficiency in energy transfer it is easy to see why the classic "Trophic Pyramid" describes the relationship between biomass in the trophic levels of a food web. An example is given below.

With this understanding, A) Calculate the amount of tuna biomass that would be generated given the following phytoplankton production and trophic structure (number of trophic levels) in an open ocean food web that has an average transfer efficiency of 12% and ends with Tuna as the apex predator. Given that large Tunas are being captured weighing 500 lbs, what volume of seawater and its accompanying primary production are needed to produce a fish of this size?

One way of representing this relationship graphically is to plot the production value versus the trophic level of this data. If this is done you would observe the following figure. What type of relationship (curve) is indicated here.

B) Why is this figure hard to interpret. What could you do to transform the data to generate a more easily understood figure? Plot on the following figure the results of: log (Production) versus trophic level. What happens to the figure? Why does this happen?

While reductions in energy and biomass (carbon) occur during the transfer between trophic levels, some substances can actually increases in concentration with trophic transfers. This is especially problematic with pollutants. Biomagnification is a process by which pollutants are accumulated to higher concentrations in predatory species due to consumption of many smaller organisms with lower concentrations. Biomagnification does not stop with the apex predators in the marine environment but often terminates with higher terrestrial predators, including birds and humans.

Trace amounts of mercury are soluble in seawater. In seawater, bacteria convert mercury to methyl mercury, a more toxic form. Fish absorb methyl mercury from water through their gills and food. Larger predatory fish are exposed to higher levels of methyl mercury from their prey. Methyl mercury binds tightly to the proteins in fish tissue. Cooking does not appreciably reduce the methyl mercury content of the fish. If the biomagnification factor describes the increase in concentration of pollutant as follows:

Biomagnification factor* =

[concentration of pollutant in higher trophic level / concentration of pollutant in lower trophic level]

* - this factor also applies to the concentration in the first trophic level versus the concentration in seawater

C) Then determine the following concentrations of methyl mercury as a function of trophic level. What is the biomagnification factor? The use of the biomagnification factor assumes that all of the food source for the upper trophic level comes from the underlying trophic level. What factors do you think would be important in estimating the potential impact to humans?

Methyl Mercury has been involved in a tragic mass poisoning of humans in Minamata, Japan. This case represents the most dramatic and emotionally moving examples of industrial pollution in history. To see a summary of this event click on the link below.

"Minamata Disease"

References:

1) Pimm, S.L., Lawton, J.H., and Cohen, J.E. 1991. Food web patterns and thier consequences. Nature 350:6320 669-674

2) Libes, S. 1992. An Introduction to Marine Biogeochemistry. John Wiley & Sons Inc., New York. 734 p.

3) FDA Consumer, September, 1994., http://foodsafety.org/il/il104.htm

4) The Poisoning of Minamata, http://www2.utep.edu/~allchin/ships/ethics/minamata.htm

Author: Eric Koepfler --- eric@coastal.edu
Last up-dated, December 14, 1998